1. Field
This invention relates generally to a seed beam source for generating an optical seed beam for a fiber laser amplifier and, more particularly, to a seed beam source for generating an optical seed beam for a fiber laser amplifier, where the seed beam source interleaves a plurality of seed beams having different wavelengths into a single seed beam.
2. Discussion
High power laser amplifiers have many applications, including industrial, commercial, military, etc. Designers of laser amplifiers are continuously investigating ways to increase the power of the laser amplifier for these applications. One known type of laser amplifier is a fiber laser amplifier that employs seed beams, doped fibers and pump beams to generate the laser beam in a manner well understood by those skilled in the art. Typically, a high power fiber laser amplifier employs a fiber that has an active core diameter of about 10-20 μm or larger. Modern fiber laser amplifier designs have achieved single fiber power levels up to 10 kW. Some fiber laser systems employ multiple fiber laser amplifiers and combine them in some fashion to provide higher powers.
Fibers used for high power fiber lasers of the diameter mentioned above support multiple transverse propagation modes, but are typically used to generate an output beam only in the lowest order fundamental mode LP01, which is near Gaussian. A fundamental issue inhibiting further scaling of LP01 mode output power from these fibers beyond about 1.5 kW is the threshold onset of power transfer to higher order modes (HOMs) from the fundamental mode. As is well understood in the art, light propagating in the lowest order fundamental mode typically has higher beam quality, where light that propagates in the high order modes incurs a reduction in spatial beam quality. This effect has inhibited the development of multi-kW, low-nonlinearity, highly coherent fiber amplifiers with a single-transverse mode output required for various applications.
Optical power transfer of laser light to HOMs generally occurs as a result of the formation of a moving long-period grating (LPG) in the fiber core refractive index that is written by the interference pattern between the fundamental mode LP01 and the next higher order mode LP11. Numerous experimental studies have shown that the mechanism for such a refractive index change in the fiber leading to formation of the LPG is thermal. More particularly, light propagating in the multiple modes will interfere with each other in the fiber creating an optical intensity grating, where the period of the grating caused by the interference is a characteristic of the fiber modes and optical wavelength. The optical interference creates thermal variations through the volume of the fiber depending on the particulars of the interference between the light beams in the different modes, which changes the index of refraction of the fiber core, and which causes the light to scatter. The scattered light is matched in phase, which causes a transfer of power from the fundamental mode to higher order modes. As optical output power increases, the LPG amplitude increases, and optical coupling builds up exponentially, eventually reaching a threshold level above which optical power dynamically fluctuates between modes. The dynamic fluctuation in modal powers is consistently observed to occur on time scales corresponding to the thermal diffusion time across the fiber core, typically about 1 ms for 20 um class core diameters in silica fibers.
The dynamic nature of the power transfer suggests that the propagation mode transfer mechanism could be inhibited by dynamically changing the phase of the LPG relative to that of the light beams on a similar or faster time scale than the thermal diffusion time. This concept has recently been verified experimentally by dynamically changing the relative amplitude and phase in a super-position of launched modes in a closed-loop servo-configuration.
Another approach to solve this problem of light propagating into higher order modes may be to substantially broaden the seed beam linewidth. Because of dispersion of the effective index of refraction difference between modes, the LPG spatial frequency will depend on the seed beam wavelength. With sufficiently broad seed beams, the LPG will wash out on a length scale comparable to the gain length, and thus prevent coherent HOM coupling. Conceptually, this is similar to standard methods used for mitigation of stimulated Brillouin scattering (SBS). However, calculations based on the intermodal dispersion suggest that the linewidths required for significant suppression appear to be much broader than for SBS on the order of one or more nanometers.
With continuous broad nm-class seed beam linewidths, it can be difficult to manage the evolution of the spectral phase and amplitude as the seed beam propagates through the high power amplifier chain. If spectral phase and amplitude are not maintained, either due to dispersion or multi-path interference affects, then an initially pure frequency-modulated (FM) source will partially convert to an amplitude-modulated (AM) source, i.e., the intensity will vary in time. This can, in turn, lead to nonlinear phase modulation from the Kerr nonlinearity that can broaden the output seed beam linewidth or reduce the coherence of the output beam, thus impeding further scaling via beam combining.